The effects of atriopeptin III (AP III) were determined on agonist-induced [i.e., 10(-4) M norepinephrine (NE)] and depolarization-induced (80 mM K+) contractions of isolated mesenteric resistance vessels (ID approximately 100 microns) from spontaneously hypertensive rats (SHR) and from normotensive control Wistar-Kyoto (WKY) rats. The vessels from both groups, when activated by 80 mM K+, were unaffected by AP III. However, activation of WKY vessels by 10(-4) M NE (both phasic and tonic contraction) was inhibited quite effectively and potently by AP III, whereas that in SHR vessels was much less inhibited. In the WKY rat vessels, the concentration of AP III that inhibited contraction by 50% for NE-induced phasic tension was 3.1 +/- 1.3 nM, whereas in SHR vessels it was nearly 1 microM. Comparison of AP III inhibition of NE-induced phasic tension to that at 5 min of activation (tonic tension) indicated that the tonic contractions were less sensitive to AP III than the phasic contractions in the vessels from both strains. A similar experiment indicated that AP III was a potent inhibitor of agonist-induced activation in a human renal resistance vessel (ID 125 microns) and that this vessel depended virtually completely on extracellular Ca2+ for NE-induced contraction. These studies contrast with earlier reports (1, 30) that similar peptides inhibited tension only in rat renal resistance vessels and not in resistance vessels from other vascular beds. The decreased sensitivity and efficacy of AP III in inhibiting tension in SHR compared with WKY mesenteric resistance vessels is discussed in the context of the etiology of spontaneous hypertension.